It is known in automotive vehicle control systems to utilize wheel speed information which is processed in a computer for operating a control such as antilock brakes or traction control. Frequently the wheel speed of all four wheels or of both front wheels and the average of the rear wheels are used for control purposes. In such cases three or four variable reluctance wheel speed sensors are fed to the computer control, each sensor having a separate channel for processing the wheel speed. The processing may include sensor and channel diagnostics, amplification, square wave generation, A/D conversion, and determination of each wheel speed. At some point in the process the signals are digitized and fed to the computer.
To the extent that the channels are separate, it is relatively easy for the computer to diagnose faulty operation in a sensor or channel: when the wheel speeds are expected to be equal, the channel outputs are compared and any deviant channel has an output different from the others. Verifying the processing occurring within the computer is more difficult. It has been proposed to use two microprocessors running in parallel so that one can be a check for the other. However, such large scale redundancy is expensive in terms of the amount of space required on a computer die.
To upgrade antilock brakes and traction control systems, it is here proposed to include on the computer die functions which previously have been accomplished separately. Yet because of the need to conserve space on the silicon die, the processing must be done efficiently and in some cases with less circuitry than used in more conventional systems. One feature is to multiplex the incoming sensor signals and process the signals through only one channel instead of three or four. This presents some challenges, for example, diagnostics of the external sensors, harnesses and processing channel no longer can rely on comparing one channel to another. New diagnostics, then, are needed not only for external circuits but for internal operations. An advantage to incorporating the input functions on the same die as the computer is that the computer is readily available for assisting with some of the processing or the diagnostics, affording some amenities not previously practical.
To afford digital wheel speed information to the control it is desirable to first form digital square waves, corresponding in frequency to each analog input signal, from which wheel speed can be calculated. Thus means for simply and reliably forming the several square waves using a minimum of die space is needed. Due to sensor runout and sometimes a damaged sensor tooth, the analog input signal is not a uniform sine wave, and special measures are needed to correctly convert to a square wave.
An anti-lock brake system for a front wheel drive vehicle usually has a separate sensor on each of the four wheels. In that case four square waves of similar characteristics are generated. However rear wheel drive vehicles may have separate sensors for front wheels and a single sensor for the rear wheels, perhaps driven by the drive shaft, and the rear sensor frequency is approximately twice that of the front sensors for the same wheel speeds. It is desired, therefore, to accommodate both kinds of systems with the same controller, but with a simple software switch for adapting to three sensor or four sensor configurations.